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Although rare, explosions and flash-over fires from methane accumulation
can and do occur in hog barns. The risk of methane fires and/or
explosions in swine facilities increases due to various operational
and design factors. This Factsheet discusses managing the risk
of methane accumulation in swine facilities.

Methane

Methane is produced by a family of bacteria called methanogens.
It is odourless, colourless and non-toxic. However, in concentrations
greater than 50%, it will act as a simple asphyxiate (i.e., its
presence results in suffocation).

Methane is slightly soluble in liquid. Its low solubility in
water suggests that manure canít store methane in the liquid phase,
but a sufficient amount is stored in the form of small bubbles
attached to manure particles. These methane bubbles are easily
released when the manure is mixed.†

With liquid manure storage systems, always assume that methane
gas is being produced. Research shows that even in cold weather,
methane bacteria will still produce up to 16 L of methane per
cubic metre of manure per day. In warm weather, the bacteria produce
up to 30 L of methane per cubic metre of manure per day.

Methane has a specific gravity of 0.5 compared to air 
half the weight of air  so it will always rise up, collecting
into storage headspace and unvented areas.

In an unvented area (closet, feed room, office or hallway) containing
methane, an ignition source (switch, motor, welder, grinder, pilot
light) can trigger a flash-over or explosion and lead to a fire.
To minimize the risk of accumulating methane, continually ventilate
all barns, or rooms within barns, that have liquid manure storage
or transfer pits, before a flammable level is reached.

Managing methane accumulation

The majority of swine facilities in Ontario produce and store
liquid manure for long periods of time within a building that
is often divided up into rooms or sections. These rooms or sections
are emptied of all livestock but left with the accumulated manure
underneath until they are needed again.

Methane will collect under the ceiling of these areas and divide
into three flammable limits:

an area above the flammable level at >15% methane mixed
with ambient air

a middle zone that is at flammable level, between 5% to 15%
methane mixed with ambient air

a zone that is on the bottom of the flammability levels at
a mixture of <5% methane mixed with air

Figure 1.
All rooms over manure storages should have ventilation operating
at all times.

Any source of ignition is a threat, but the greatest threat is
at the critical zone  the middle layer. The most effective
management practice is to provide continuous ventilation throughout
the barn. Even if the barn is empty of animals, where liquid manure
is present, provide at least three uniform air changes or more,
per day, to properly ventilate the facility.

Storing Manure

Any sudden release of methane gas can overwhelm the ventilation
system of a barn.

Double-pitted barns are designed so that the short-term storage
is located above the long-term storage, and manure is drained
periodically to the lower storage via pull plugs. If the lower
storage is unvented to the exterior, the methane builds up in
the headspace. When the upper short-term storage is drained, this
methane is displaced by the incoming manure, moving up to the
short-term storage.

Furthermore, if the pull plugs are left out, all the methane
rises up into the room above and can cause an explosion or fire
(Figure 2). The easiest solution for double-pitted
barns is to ventilate the lower pit to the exterior so methane
is removed as it forms.

Figure 2.
This open pull plug hole is the suspected source of methane
from the lower pit that resulted in a nursery barn fire.

Moving Manure

Some facilities move manure using a transfer pit under the hallway
to an exterior pump-out pit. In some barns, there is a solid baffle
between the transfer pit and the pump-out pit, and the transfer
pit is not vented to the exterior.

Once the level of the manure reaches the bottom of the baffle,
the unvented transfer pit becomes hydraulically sealed (Figure
3). Any additional manure will result in a pressure build-up
in the headspace of the transfer pit. If there is a drain hole,
cover plate or floor drain, the methane in the headspace will
shoot out of the hole(s), under pressure, into the hallway or
the room above. If the ventilation system is unable to handle
this sudden inflow of gas, the flammable level is quickly reached.

Figure 3. Pump-outs
with baffles will cause the head spaces in unvented storages
to become pressurized once the manure level rises above the
bottom of the baffle.

Foaming Manure

Foaming manure is a combination of gas, methane bubbles, a surfactant,
soap and a stabilizer: filamentous methanogens bacteria.

Dr. Bo Hu, University of Minnesota, is part of a team that has
worked on this issue since 2010. One theory is that soap is being
formed from incompletely digested fats in the pigís diets. These
fats form long-chain fatty acids, which are converted to surfactants
and then soap by the action of the bacteria present in the manure.
The use of dried distillers grains with solubles (DDGS) for feed
is suspected as the cause, although a survey in Ontario found
that some barns have been experiencing foaming issues since before
the common use of DDGS.

The methane portion of the foam makes up 50%70% of the
total volume. The foam itself can be several feet thick (Figure 4).
If the foam is rapidly broken up by aggressive agitation or pressure
washing, methane is suddenly released, overwhelming the ventilation
system. Recently, many swine barn fires in the U.S. Midwest have
been caused by manure foam.

Figure 4.
A manure pit cover that was pushed off by manure foam.

Summary

Always assume methane is being produced in liquid manure systems.

Ensure that ventilation is provided to all areas of the barn,
including individual rooms.

Vent all storage and transfer pits to the exterior so as to remove
the methane as it is being produced and eliminate pressure build-up
in the headspace.

Treat any sign of foaming manure with extreme caution. Under
no circumstances should the manure foam be aggressively knocked
down or agitated.

These best management practices will greatly reduce the risk
of methane accumulation in swine facilities.

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